Microphones



S p .13,1955 R. H. RACKHAM ETAL v 2,717,932

MICROPHONES Filed Aug. 8, 1951 2 Sheets-Sheet 1 24 v 24 fiwentom RONALD HASTINGS RACKHAM, G-TERENCE BRIA LIVINGSTONE,

Sept. 13, 1955 R. H. RACKHAM ET AL MICROPHONES 2 Sheets-Sheet 2 Filed Aug. 8, 1951 &

ttorneysf HHHHHHHHHMHH HHH \V Inventors RONALD HASTINGS RACKHA M TERENCE BRIAN hhhh 3/ United States Patent MICROPHONES Ronald Hastings Rackham and Terence Brian Livingstone, West Norwood, London, England, assignors to Tannoy Limited, West Norwood, London, England, a British company With the increasing public use of microphones, such as by entertainers, public speakers and others, the need has developed for microphones for public use wh1ch will be as unobtrusive as possible when placed In front of the user, and especially the users face, in order to avoid distracting the audience, and indeed the user also, from the users performance or delivery. Such an unobtrusive microphone is particularly desirable for entertainment purposes.

An object of the present invention is to provldean improved microphone which will meet these and similar requirements.

According to the present invention, there 1s provided a microphone comprising a casing in which an electroacoustic transducer of the pressure microphone type is housed opposite an aperture in the casing to which a tube extending from the casing is attached at one end so as to be acoustically coupled at that end to the transducer to form an acoustic transmission line thereto, the tube being open at its other end and provided therem with acoustic damping means for preventing sound reflections from the open end of the tube.

A microphone embodying the invention may be constructed as a stand microphone such as is required for standing on a floor, table or the like. In such embodiments, the casing and tube form a stand, of which the casing constitutes the base and the tube constitutes the stem. A microphone according to the invention may, however, also be constructed as a portable hand microphone. In contrast to an ordinary stand microphone or hand microphone in which the stem of the stand, or the handle, as the case may be, has the purely mechanical function of supporting the transducer at the requisite position in front of the user, the function of the tubein a microphone according to the present invention is not to support the transducer, since that is housed in the casing or base of the device, but to serve for transferring the transducer diaphragm, in eifect, to the open end of the tube. The non-directional characteristics of the pressure type of transducer are retained with this arrangement, which behaves as though a miniature pressure microphone were at the open end of the tube.

According to a feature of the invention, the transducer is mechanically isolated from the casing and the tube, so as to prevent transmission of mechanical vibrations from the tube and casing to the transducer. This isolation may be achieved by providing sponge rubber or similar vibration-absorbing packing in the mounting of the transducer in the casing, so that the packing is interposed between the transducer and the parts of the casing to which it is fixed. If desired, similar packing may be provided between the tube and the casing in order to isolate these two parts against the transmission of mechanical vibrations.

The tube may be made of any suitable material, and as desired or according to the purpose for which the device is to be used, the tube may be rigid, or composed of telescopic rigid sections, or composed of 2,717,932 Patented Sept. 13, 1955 ice universally jointed rigid elements so as to be flexible as a whole, of such a character as to stay put and be self-supporting in any position to which it is adjusted, so long, however, as its material in every case is sulficiently rigid, in the sense of possessing suflicient mass, to avoid the production of acoustic resonances by mechanical vibration of the tube material longitudinally or laterally. As one example, for a stand microphone the tube constituting the stern of the stand may comprise two telescopic rigid sections, viz., a lower section of, e. g., /2" outside diameter and, e. g., 3 ft. in length secured to the casing forming the base of the stand, and an upper section of, e. g., 2 ft. in length telescoped within the lower section. The length of the tube is not critical and may be varied in the ratio of approximately 2:1 (as in the case of a telescopic tube) without need for altering the amount of damping provided at its open end. The amount of damping provided is that which is sufficient effectively to damp out the fundamental resonance of the tube which is dependent on its length. The damping means may consist of a plug, disc, or nodule of cotton wool, acoustic felt, or other customary acoustic damping material.

The transducer housed in the casing may be a normal type of moving coil pressure microphone provided with a throat, the clearance of the throat on the microphone being made small, preferably not greater than .005". However, in order that the desired ferquency response may be obtained from the electro-acoustic transducer, together with its associated tube, it is essential that this transducer should have a frequency characteristic which rises sharply in the upper frequency range. In order that this may be satisfactorily achieved, a small cavity, which is formed by the spacing between the diaphragm and pole-tip assembly, is released to the main cavity and its associated damping through a very small orifice of a controlled size. In the event of there being a very large diiference in the length of the tube to be employed for diflerent uses of the same transducer unit, the main microphone damping may have to be adjusted.

The transducer is mounted with sponge rubber packing, as above described, in the casing opposite the aperture therein over which one end of the tube is fitted opposite the throat. For attaching the tube, it may be provided with a flange at its fixing end which abuts, if desired with the interposition of an annular sponge rubber packing, against the end of an externally screwthreaded neck on the casing around its aperture, a gland nut engaging over the flange being screwed on to the neck to secure the tube to the casing. If desired, the easing may also house matching equipment, such as a transformer, for the transducer.

Over the open end of the tube is fitted a domed screen, or so-called mushroom, for the purpose known as der plopping, that is to say, for preventing the effect caused by D. C. air impulses on the open end of the tube, such as are produced by explosive consonants, which have the effect of producing a plopping sound in the reproducing loudspeaker. This mushroom is made of silk or similar sheer fabric laid over a domed metal gauze grille provided for lending mechanical support to the fabric. It will be understood, of course, that a gap is left between the underside of the mushroom and the open end of the tube. To maintain the high frequency response of the complete microphone, it is desirable to flare the open end of the tube, in order to increase its effective pick-up area.

However, a microphone according to the invention is constructed, whether as a stand or hand microphone or otherwise, in every case, by means of the invention, the actual transducer is positioned away from the users face and the user merely holds or has in front of him the open end of the tube, which behaves as through it were a miniature pressure microphone. For hand microphones, the casing housing the transducer, and also its ancillary equipment if desired, may be arranged for portability in any convenient way and position on the users person, so that the tube, which may for this use be flexible, can be held in the hand to bring its open end in the position which with normal types of hand microphone, is occupied by the microphone itself.

In order that the invention may be more clearly understood, one embodiment thereof will now be described by way of example with reference to the accompanying drawing in which:

Fig. 1 shows a view, partly in section, of a microphone constructed according to the invention and Fig. 2 shows a view, partly in section, of one form of transducer unit.

The microphone comprises a circular dome-shaped base or casing 1, which may for example be cast from metal or moulded from synthetic resin material, and which is provided with an aperture 2 in the centre of the domed surface. The electro-acoustic transducer unit 3 of the pressure microphone type is mounted within the base 1, being located in alignment with the aperture 2 by a sponge rubber locating ring 4 and held in position by a sponge rubber block 5 located between the bottom of the unit 3 and a plate member 6 which covers the underside of the base 1 and to which it is attached, for example by screws or in any other convenient manner. The base may also house a matching transformer 7, the external cable or connections being made through a socket member 13 also fixed to the base 1.

The top of the base is formed with an upstanding externally threaded neck portion 8, which is coaxial with the aperture 2. Within the neck 8 is fitted one end of a section of tube 9, the end of the tube abutting against a shoulder 19 provided within the neck portion and the tube being secured by means of a gland nut 11 which is screw-threaded over the neck portion 8, a clamping ring or spring washer 12 being interposed between the neck portion 8 and the nut 11 securely to hold the end of the tube 9.

Within the upper end of the tube 9 telescopically slides the lower end of a further tube 14, such that the tube 14 can be telescoped partially within the tube 9. The upper end of the tube 9 is externally threaded and is provided with a locking ring 15 and a spring washer 16, which, when tightened, securely hold the tube 14 in relation to the tube 9. The outer surface of the locking ring 15 is knurled so that this may be easily turned by the fingers and loosened or tightened when it is desired to adjust the height of the tube 14.

Tube 14 is curved at its upper end 17, to which is fitted a flared end portion 18. The mouth of the end portion 18 is covered with a gauze grille 19 backed by a piece of silk 20, the gauze and silk being mounted in a ring 21 which is spaced away from the end of the flared mouth by spacer members 22 so that an annular gap is left between the underside of the ring 21 and the mouth of the flared end portion 18. Damping means comprising a disc of acoustic felt 23 is provided at the upper end of the tube 14, being held in position by the flared end portion 18.

The microphone described is intended as a floorstand microphone and the plate member 5 is provided with rubber feet 24. The curving of the tube 14 brings the flared end 18 conveniently positioned near to the mouth of a speaker. The stem comprising the telescopic tubes 9 and 14 may be adjusted to alter the height of the microphone for different users.

The transducer unit 3 may be constructed as shown in Fig. 2 of the drawing, which construction provides a transducer having a frequency characteristic which rises sharply in the upper frequency range. The transducer comprises a cylindrical magnet 25, provided with a pole piece 26, and surrounded by a magnet cup 27. The circular diaphragm 28 which is made from Duralumin foil, extends over the face of the pole piece 26, and its periphery is clamped between a moulded Bakelite ring 29 secured to the magnet cup 27 and an annular throat member 3t), by means of nuts and bolts 31. Annular spacing rings 32 are arranged between the molded synthetic resin ring 2) and the periphery of the diaphragm, and between the throat member and the periphery of the diaphragm. The speech coil 37 supported from the diaphragm is located in the annular magnetic gap between the pole piece 26 and the magnet cup 27. v

To obtain the rising frequency characteristic the small cavity 33 between the pole piece and the diaphragm is connected to the main cavity in the casing through a small gap 35 provided between the inner edge of a sealing riu g 34-, which is disposed between the cylindrical magnet 25 and the magnet cup 27, and the outer wall of the cylindrical magnet 25. The space between the magnet and the magnet cup on the side of the sealing ring 34, remote from the speech coil is packed with an acoustic felt damping material 36.

The frequency response can be adjusted by adjusting the size of the gap between the sealing ring 34 and the outer wall of the cylindrical magnet 25.

Whilst a particular embodiment has been described, it will be understood that various modifications may be made without departing from the scope of the invention. For example, the base housing the transducer may be so styled as to be suitable for use as a table or desk stand, and may, with the addition of extension pieces, and/ or of longer or more tubes, be adapted for use as a floor stand microphone.

We claim:

l. A microphone comprising a casing provided with an aperture, a pressure-operated electro-acoustic transducer within said casing opposite said aperture, a tube extending from the aperture in the casing matching means between said transducer and said tube, a screen comprising a layer of fabric fitted over the open end of said tube, a gap between said screen and the open end of said tube, and acoustic damping means extending transversely across said tube.

2. A microphone comprising a casing provided with an aperture, a pressure-operated electro-acoustic transducer within said casing opposite said aperture, a tube extending from the aperture in the casing, said tube having a flared portion at its open end matching means between said transducer and said tube, and acoustic damping means arranged transversely across said tube adjacent said flared end.

3. A microphone comprising a casing provided with an aperture, a pressure-operated electro-acoustic transducer within said casing opposite said aperture, a tube comprising at least two telescopic sections extending from the aperture in the casing, vibration-absorbing means supporting said transducer within said casing opposite said aperture matching means between said transducer and said tube, and a disc of acoustic damping material arranged transversely across said tube.

4. A microphone comprising a casing provided with an aperture, a pressure-operated electro-acoustic transducer within said casing opposite said aperture, 3. tube comprising at least two telescopic sections extending from the aperture in the casing, a screen comprising a grille supporting a layer of fabric fitted over the open end of said tube, a gap between said screen and the open end of said tube matching means between said transducer and said tube, and acoustic damping means arranged transversely across said tube.

5. A microphone comprising a casing provided with an aperture, a pressure-operated electro-acoustic transducer within said casing opposite said aperture, a tube extending from the aperture in the casing matching means between said transducer and said tube, vibration-absorbing means supporting said transducer within said casing opposite said aperture, a screen comprising a layer of fabric fitted over the open end of said tube, a gap between said screen and the open end of said tube, and acoustic damping means arranged transversely across said tube.

6. A microphone comprising a casing provided with an aperture, a pressure-operated electro-acoustic transducer within said casing opposite said aperture, a tube comprising at least two telescopic sections extending from the aperture in the casing matching means between said transducer and said tube, vibration-absorbing means supporting said transducer within said casing opposite said aperture, a screen comprising a grille supporting a layer of fabric fitted over the open end of said tube, a gap between said screen and the open end of said tube, and a disc of acoustic damping material arranged transversely across said tube.

7. A microphone comprising a casing provided with an aperture, a pressure-operated electro-acoustic transducer within said casing opposite said aperture, a tube comprising at least two telescopic sections extending from the aperture in the casing matching means between said transducer and said tube, vibration-absorbing means supporting said transducer within said casing opposite said aperture, a flared end portion fitted to the open end of said tube, and a disc of acoustic damping material arranged transversely across said tube.

8. A microphone as claimed in claim 7 wherein the transducer unit has a frequency response which rises sharply in the upper frequency range.

9. A microphone comprising a casing provided with an aperture, a pressure-operated electro-acoustic transducer within said casing opposite said aperture, a tube extending from the aperture in the casing matching means between said transducer and said tube, vibration-absorbing means supporting said transducer within said casing opposite said aperture, said tube having a flared portion at its open end, a screen comprising a layer of fabric fitted over the mouth of said flared end portion, a gap between said screen and the mouth of said flared end portion, and a disc of acoustic damping material arranged transversely across said tube.

10. Apparatus as claimed in claim 9, wherein the electro-acoustic transducer comprises a moving coil pressure microphone comprising a casing, an annular throat member attached to said casing and forming said matching means, a magnetic pole assembly having an annular air gap, housed within said casing, a diaphragm, a coil attached to said diaphragm and positioned in said air gap, and a small orifice connecting the cavity formed by the spacing between said diaphragm and the central pole surrounded by said air gap with the cavity between said microphone and said casing.

11. Apparatus as claimed in claim 9, wherein the electro-acoustic transducer comprises a moving coil pressure microphone comprising a casing, an annular throat memoer attached to said casing and forming said matching means, a magnetic pole assembly comprising a central magnet and a surrounding magnet cup housed within said casing, an annular air gap between said centralmagnetand said magnet cup, a circular diaphragm, a coil attached to said diaphragm and positioned in said air gap, a sealing ring behind said annular air gap and between said central magnet and said magnet cup, a small gap between the inner edge of said sealing ring and the outer wall of said central magnet and acoustic damping means between said central magnet and said magnet cup adjacent said sealing ring on the side of said sealing ring remote fromsaid speech coil.

References Cited in the file of this patent UNITED STATES PATENTS 1,584,038 Kornsweet May 11, 1926 1,818,654 Steuart Aug. 11, 1931 2,010,566 Schnetz Aug. 6, 1935 2,238,741 Laulfer Apr. 15, 1941 2,239,503 Kreuzer Apr. 22, 1941 2,293,078 Proctor Aug. 18, 1942 2,485,405 Olney et a1. Oct. 18, 1949 

